Semiconductor integrated circuit

ABSTRACT

A semiconductor integrated circuit is disclosed in which a power MOSFET supplies a squib of automobile air bag systems with load current. The power MOSFET Q 1  provides squib Z L  with the load current, and load current signal which outputs from shunt resistor R s  is provided an operational amplifier consisting of transistors Q 4  -Q 10  with a negative feedback signal, so that the load current to be supplied to the squib Z L  is restricted. The negative feedback operation is interrupted by load current function interruption signal which inputs to terminal T 6 . A circuit which consists of two current mirror circuits composed of transistors Q 4  -Q 10  and constant current source I 4  supplies the operational amplifier with constant current to interrupt the feedback operation.

FIELD OF THE INVENTION

This invention relates to a semiconductor integrated circuit, and more specifically, relates to a semiconductor integrated circuit which is employed in automobile air bug systems and the like.

DESCRIPTION OF THE PRIOR ART

Recently, automobile air bag systems tend to be installed in more and more automobiles, in which the air bag is adapted to expand automatically as soon as collision accident takes place, and protects driver or crew in the collided automobile. In such air bag systems, when an impact which exceeds a predetermined level is applied to automobile body, an acceleration sensor detects the impact, and expands the air bag by causing an electric current to flow through an initiating explosive (called "squib") for an explosive compound to ignite it.

Such air bag systems, as shown in FIG. 6, consist of DC-DC converter 1 which steps up output voltage of battery B installed in an automobile up to a predetermined voltage, capacitor C_(B) with large capacitance connected between output terminals T₁ and T₂ of the DC-DC converter 1, and squib driver circuit 2, with a squib Z_(L) being connected between the output terminals T₄ and T₅ as a load of the squib driver circuit 2. The DC-DC converter 1 and the squib driver circuit 2 are integrated into the same integrated circuit.

In FIG. 6, the capacitor C_(B) with large capacitance connected between the output terminals T₁ and T₂ is always charged to an output voltage V_(CC) of the DC-DC converter 1 while the automobile is in operation. Accordingly, even if the battery B is destroyed or power supply lines 3, 4 from the battery B to the DC-DC converter 1 are cut off by an accident, electric charge which is charged in the capacitor C_(B) discharges through the squib driver circuit 2, and the capacitor C_(B) can supply the squib driver circuit 2 with power. Namely, the capacitor C_(B) functions as a backup battery for the squib driver circuit 2.

Incidentally, in such air bag systems, the squib driver circuit 2 is required to flow, through the squib Z_(L), current more than a certain predetermined amount for at least a predetermined period. However, when the squib driver circuit 2 supplies the squib Z_(L) with current without any limitation, the output voltage of capacitor C_(B) as the backup power supply will drop in a short period of time. Accordingly, when a current over predetermined level flows through the squib Z_(L), it becomes necessary to make the voltage drop delay by limiting the load current.

FIG. 7 shows an example of such squib driver circuit with the load current limit function. In FIG. 7, transistor Q₁ is a P-channel power MOSFET which controls a current supplied to the squib Z_(L). Transistors Q₄ -Q₁₀ constitute an operational amplifier together with constant current source I₁, resistors R₁, R₂, R₄ and capacitor C₁. The transistor Q₁ constitutes a power amplifying stage in the operational amplifier. Gate of the transistor Q₁ is connected to collector of the transistor Q₆ in the operational amplifier, shunt resistor R_(S) is connected between source of the transistor Q₁ and the power supply V_(CC). And the squib Z_(L) is connected between an output terminal T₄ connected to drain of the transistor Q₁ and another output terminal T₅ connected to the ground. Base of the transistor Q₇ in the operational amplifier is connected to node J_(A) between source of the transistor Q₁ and the shunt resistor R_(S). Base of the transistor Q₈ is connected to node J_(B) between the resistors R₁ and R₂ which are connected to power supply V_(BG) in parallel.

The transistor Q₃ has its collector connected to collector of the transistor Q₄ in the operational amplifier. The transistor Q₃ has its base connected to collision sense signal input terminal T₃, and its emitter connected to the ground. Resistor R₃ is connected between base of the transistor Q₈ in the operational amplifier and collector of the transistor Q₂. The transistor Q₂ has its base connected to input terminal T₆ to which is supplied a signal for canceling load current limit function, and its emitter connected to the ground. A circuit consisting of the transistor Q₂ and the resistor R₃ constitutes a load current limit function cancel circuit for canceling the current limit function in the squib driver circuit 2.

The squib driver circuit 2 operates as follows. First of all, assume that the transistor Q₂ and the resistor R₃ are not present. When the transistor Q₁ is OFF, the potential on node J_(A) is equal to the potential V_(cc) of power supply line, and the potential on node J_(B) is equal to reference potential V_(REF) which is determined in accordance with the power supply V_(BG) and the resistors R₁, R₂. Consequently, the base potential in transistor Q₇ is higher than the base potential of the transistor Q₈, and the operational amplifier do not operate as a negative feedback amplifier, but operate as an comparator. Accordingly, when the transistor Q₁ is OFF, the squib driver circuit 2 shown in FIG. 7 is equivalent to a circuit shown in FIG. 8, and the operational amplifier operates only as a constant current load for the transistor Q₃. Namely, when the potential on collision sense signal input terminal T₃ is in logical high (hereinafter referred to as "H") and the base potential in transistor Q₃ is "H" so that the transistor Q₃ is switched on, the transistor Q₆ is switched off, with the result that the P-channel transistor Q₁ also turns off and current does not flow through squib Z_(L).

Under the above circumstances, when the potential on the collision sense signal input terminal T₃ has switched to logical low (hereinafter referred to as "L"), the P-channel transistor Q₁ turns on so that current flows through the squib Z_(L). When the current flows through the squib Z_(L), the current also flows through the current sense resistor R_(S) called shunt resistor, the potential V_(A) on node J_(A) decreases from the power supply potential V_(CC). When the potential V_(A) on node J_(A) drops approximately to the reference potential V_(B) on node J_(B), and further the potential V_(A) drops to a lower potential than the potential V_(B), the operational amplifier enters into its linear operation region. As a result, the operational amplifier operates as a negative feedback amplifier and a base current in transistor Q₆ may decrease.

As stated above, when the base current in transistor Q₆ decreases, it becomes difficult for this transistor Q₆ to drive the load resistor R₄ sufficiently, and the collector potential in transistor Q₆ increases. Thus, the gate potential in transistor Q₁ whose gate is connected to collector of the transistor Q₆ begins to increase, and the current which flows through the P-channel transistor Q₁ is limited. Thus, potential difference between source of the transistor Q₁ and drain thereof increases, and the voltage applied to the squib Z_(L) decreases.

When the current which flows through the transistor Q₁ is limited, the potential difference between drain of the transistor Q₁ and source thereof increases, and a power loss in the transistor Q₁ increases abruptly. When the power loss exceeds a standard value for the transistor Q₁, there is a possibility that the power transistor Q₁ may be destroyed.

Thus, in the squib driver circuit 2 shown in FIG. 7, until the power supply voltage V_(CC) decreases to be equal to or lower than a predetermined value V_(LC), the potential on input terminal T₆ for canceling the load current limit function is made logical "H" to switch on the transistor Q₂ and to decrease the potential V_(B) on node J_(B) from the reference potential V_(REF). As a result, even though the current supplied from the transistor Q₁ to the squib Z_(L) becomes larger than the predetermined value I_(CL), the current which flows through the transistor Q₁ is not limited, and the power loss in the transistor Q₁ becomes smaller. After that, when the power supply voltage V_(CC) has decreased to the value V_(CL), the potential of control signal input terminal T₆ is made logical "L" and the transistor Q₂ is switched off. Thus, the current in the transistor Q₁ is limited as stated above. Then, because the power supply voltage V_(cc) has decreased, a potential difference between source of the transistor Q₁ and drain thereof is trivial, and the power loss in transistor Q₁ is comparatively small.

Incidentally, in the prior squib driver circuit 2 in FIG. 7, when the transistor Q₂ switches on and the current restriction is removed, a current flows through the resistances R₁, R₃ and transistor Q₂ from the power supply V_(CC) to the ground, and the current consumption in the squib driver circuit 2 increases. The above increase in the current consumption is substantially determined by the power supply voltage V_(CC) and the resistances R₁, R₃. Now, on the assumption that V_(CC) =20 volts, R₁ =5 kilo-ohms, and R₃ =15 kilo-ohms, the above increase in the current consumption is about 1 milli-ampere in accordance with V_(CC) /(R₁ +R₃).

As stated above, when the current consumption in the squib driver circuit 2 increases, the voltage in capacitor C_(B) which acts as a backup power supply in air bag system, that is, the power supply voltage V_(CC) for the squib driver circuit 2 decreases fast to that extent.

Although the increase of current consumption in the squib driver circuit 2 can be limited to small if the values of the resistors R₁, R₃ are made large, it is disadvantageous to make resistance in these resistors large for chip size in an integrated circuit. Furthermore, the voltage drop in the resistor R₁ become large in accordance with the increase of the current consumption, and a change arises in the reference voltage V_(REF).

SUMMARY OF THE INVENTION

This invention is intended to solve these problems as stated above, and a primary object of the present invention is to provide a semiconductor integrated circuit in which a current consumption taken out from backup power supply is small even though load current limit function is removed.

Another object of the present invention is to provide a semiconductor integrated circuit which can operate stably, with high reliability.

Still another object of the present invention is to provide a semiconductor integrated circuit which has a simple circuit construction and a small chip size.

And still another object of the present invention is to provide a semiconductor integrated circuit in which an operational amplifier means with a load current limit function is not restricted for setting a reference voltage.

In order to accomplish the above objects, according to one aspect of the present invention, there is provided a semiconductor integrated circuit comprising a backup power supply means, an operational amplifier means for supplying a load with a load current from a power amplifier means and having a load current limit function for limiting the load current to provide an input stage with a negative feedback signal of a load current signal corresponding to the load current, and a load current limit function interruption means for interrupting the negative feedback operation of the operational amplifier means with a interruption signal of the load current limit function, wherein the operational amplifier means and the load current limit function interruption means are backed up by the backup power supply means, the load current limit function interruption means consists of a constant current circuit means for controlling interruption of the negative feedback operation to supply the operational amplifier with a constant current.

Since the interruption of feedback operation is controlled by providing the operational amplifier with a constant current, the invention is advantageous in that the current consumption supplied from the backup power supply can be made small in case of interruption of the load current limit operation.

In the above mentioned semiconductor integrated circuit, it is preferable that the constant current circuit means comprises a constant current source, a first current control circuit for receiving a current from the constant current source and outputting a current corresponding to the current that is provided from the constant current source in accordance with the interruption signal, a second current control circuit for receiving an output current from the first current control circuit and making stop the negative feedback operation by supplying the operational amplifier with a current corresponding to the output current.

In the above mentioned case, since the feedback operation is ceased by supplying current corresponding to the output current from the constant current source through the first current limit circuit and the second current limit circuit with the operational amplifier, the present invention is further advantageous in that the interrupt operation in the load current limit operation can be performed with high reliability in accordance with the constant current source.

In the above mentioned semiconductor integrated circuit, it is preferable that the output current from the second current control circuit is supplied to an active load in a differential amplifier which constitutes an input stage of the operational amplifier.

In the above mentioned case, since the feedback operation is ceased by supplying the active load in a differential amplifier which constitutes the input stage of the operational amplifier means with current corresponding to the output current from the constant current source, the invention is still further advantageous in that the operational amplifier means with the feedback current limit operation is not restricted for setting the reference voltage, the integrated circuit design becomes easy.

In the above mentioned semiconductor integrated circuit, it is preferable that the first current control circuit and said second current control circuit consist of current mirror circuits respectively.

In the above mentioned case, since each of the first current control circuit and the second current control circuit consists of current mirror circuit, the integrated circuit can provide the operational amplifier with a constant current equal to a output current from the constant current source, and thus, the present invention is still further advantageous in that an operation of the operational amplifier in case of interruption of the load current limit is to be stable, whereby a semiconductor integrated circuit is provided which has high reliability.

In the above mentioned semiconductor integrated circuit, it is preferable that the first current control circuit consists of a resistor and the second current control circuit consists of a current mirror circuit.

In the above mentioned case, since the first current control circuit consists of the resistor, the present invention is still further advantageous in that a circuit structure in the load current limit function interruption means becomes simple, and can be small in chip size.

In the above mentioned semiconductor integrated circuit, it is preferable that the current mirror circuit consisting of the second current control circuit has leakage cut resistor.

In the above mentioned case, since the current mirror circuit which constitutes the second current limit circuit has the leakage cut off resistor, the present invention is still further advantageous in that an integrated circuit with stable operation can be obtained.

In the above mentioned semiconductor integrated circuit, it is preferable that the the constant current circuit means comprises two constant current sources, an amplifier having these constant current sources respectively as loads connected to inputs of differential amplifier constituting an input stage in the operational amplifier, a current from one of the constant current sources is bypassed by said interruption signal from an amplifier which is said one of the constant current sources as a load to cease the feedback operation by making the input stage of the operational amplifier out of balance.

In the above mentioned case, since the current from one of two constant current sources is made bypass from the amplifier which has the one of constant current sources as a load, and the input amplifier means in the operational amplifier is made out of balance and cease the feedback operation, the present invention is still further advantageous in that the circuit operation before and/or after beginning of load current limit operation becomes stable.

In the above mentioned semiconductor integrated circuit wherein it is preferable that the output stage in the operational amplifier consists of power MOSFET.

In the above mentioned case, the present invention is still further advantageous in that a circuit structure for driving the output stage becomes simple, and chip size of the circuit can be small.

The other objects, features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of preferred embodiment 1 of the present invention;

FIG. 2 is a circuit diagram of preferred embodiment 2 of the present invention;

FIG. 3 is a circuit diagram of preferred embodiment 3 of the present invention;

FIG. 4 is a circuit diagram of preferred embodiment 4 of the present invention;

FIG. 5 is a circuit diagram of preferred embodiment 5 of the present invention;

FIG. 6 is a schematic system block diagram of an automobile air bag system;

FIG. 7 is a circuit diagram of an prior squib driving circuit in an automobile air bag system; and

FIG. 8 is a drawing for describing the operation of the squib driving circuit of FIG. 7.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Hereinafter, a plurality of preferred embodiments of the invention will be described in connection with a squib driver circuit in which a P-channel power MOSFET drives a squib of air bag systems.

Preferred Embodiment 1

In a squib driver circuit 2a shown in FIG. 1, a transistor Q₁ is a P-channel power MOSFET which controls a current supplied to squib Z_(L). Transistors Q₄ -Q₁₀ constitute an an operational amplifier together with a constant current source I₁, resistors R₁, R₂, R₄ and capacitor C₁. The transistor Q₁ constitutes a power amplifying stage in the operational amplifier. The transistor Q₁ has its gate connected to collector of the transistor Q₆ in the operational amplifier, and a shunt resistor R_(s) is connected between source of the transistor Q₁ and the power supply V_(CC). The squib Z_(L) is connected between an output terminal T₄ connected to drain of the transistor Q₁ and another output terminal T₅ connected to the ground.

The transition Q₇ in the operational amplifier has its base connected to node J_(A) between source of the transistor Q₁ and the shunt resistor R_(S). The transistor Q₈ has its base connected to node J_(B) between the resistors R₁ and R₂. These resistors R₁, R₂ are connected to power supply V_(BG) in parallel. The transistor Q₃ has its collector connected to collector of the transistor Q₄ in the operational amplifier. The transistor Q₃ has its base connected to collision sense signal input terminal T₃, and its emitter connected to the ground.

Above described structure is the same as that of the squib driver circuit 2 described in FIG. 7. The squib driver circuit 2a in FIG. 1 further comprises constant current source I₄ and two current mirror circuits consisted of transistors Q₁₁ -Q₁₄ as a load current interrupt function release means. The transistors Q₁₁ and Q₁₂ constitute a current mirror circuit. Each of the transistors Q₁₁, Q₁₂ has its emitter connected to the ground and bases of the transistors Q₁₁, Q₁₂ are connected each other. The current source I₄ is connected between collector of the transistor Q₁₂ and the power supply V_(CC). The transistor Q₁₂ also has its collector connected to collector of the transistor Q₂. The transistor Q₂ has its emitter connected to the ground and its base connected to input terminal T₆ for controlling on/off of the current limitation.

The transistors Q₁₃, Q₁₄ constitute another current mirror circuit. Each of these transistors Q₁₃, Q₁₄ has its emitter connected to the power supply V_(CC), bases of the transistors Q₁₃, Q₁₄ are connected each other. The transistor Q₁₂ has its collector connected to base in the transistor Q₆. The transistor Q₁₄ has its collector connected to collector of the transistor Q₁₁ and its base.

The squib driver circuit 2a operates as follows. That is, when the load current restriction function is needed, logical "H" inputs to the input terminal T₆ for said load current restriction function release signal. As a result, the transistor Q₂ is switched on, and the current from the constant current source I₄ is bypassed to the ground by the transistor Q₂, so that the transistor Q₁₃ is switched off. The circuit in FIG. 1 then performs the same load current restriction function as that of the circuit in FIG. 7.

On the other hand, when the load current restriction function is to be released, logical "L" inputs to the control signal input terminal T₆, and each of the two current mirror circuits which respectively consists of the transistors Q₁₁, Q₁₂ and the transistor Q₁₃, Q₁₄ is biased to operate. A current value in this case is determined in accordance with the constant current source I₄, if each emitter size of the pnp transistors Q₁₁, Q₁₂ is equal to each other and each collector length of the pnp transistors Q₁₃, Q₁₄ is equal to each other, collector current of transistor Q₁₃ becomes approximately equal to I₄.

The constant current source I₄ is then required that (I₄ -I₁) is sufficient to drive the transistor Q₆. In general, in case of (I₄ -I₁)=I₁, that is, I₄ =2×I₁, the transistor Q₁₃ supplies current 2×I₁. Accordingly, in order to limit the load current, even if collector of the transistor Q₄ intends to draw in a maximum current (that is, I₁) in its ability, at least the transistor Q₆ is supplied at its base with a base current I₁, and thus, the load current restriction operation may be inhibited.

For example, in case of V_(CC) =20 V, R₄ =20 kilo-ohms, the collector current in transistor Q₆ is about 1 mA. If a current amplification factor h_(FE) in saturation of the transistor Q₆ is 10-20, the transistor Q₆ requires 50-100 μA for its base current, and therefore the currents I₁ and I₄ may be set as I₁ =50-100 μA and I₄ =2×I₁ =100-200 μA. That is, an increase in consumption current is only about 100-200 μA in the removal of the load current limit operation.

Accordingly, the squib driver circuit 2a in FIG. 1 has an advantage that the increase of the current consumption can be held down even if the load current limit operation is removed. Because the circuit 2a does not require the resistors R₁, R₂ which have large resistance as the squib driver circuit 2 in FIG. 7, its chip size can be made small, and the circuit 2a hold down the change of voltage V_(B) (the reference voltage V_(REF)) on node J_(B) based on the base current in transistor Q₈.

Preferred Embodiment 2

A squib driver circuit 2b shown in FIG. 2 has a construction that current leakage cut off resistor R_(LC) is connected between each base of the transistors Q₁₃, Q₁₄ constituting the current mirror circuit and the power supply V_(CC) in the squib driving circuit 2a shown in FIG. 1.

When the transistor Q₂ is switched on, the transistor Q₁₁ is switched off, and the current leakage cut off resistor R_(LC) is not present, base of the transistor Q₁₃ is substantially to be an open state, a leakage current (collector cut off current I_(CEO)) flows through the transistor Q₁₃. However, in this preferred embodiment shown in FIG. 2 even if the transistor Q₂ is switched on and the transistor Q₁₁ is switched off as stated above, the leakage current is held down since base of the pnp transistor Q₁₃ is connected to the power supply V_(CC) through the resistor R_(LC). Accordingly, the current consumption in the squib driver circuit 2b is held down to be smaller, and the circuit 2b can operate more stably.

Incidentally, in FIG. 2, the portions which correspond to ones in FIG. 1 are assigned the same reference numerals as ones of FIG. 1, and the descriptions of them will be omitted.

Preferred Embodiment 3

A squib driver circuit 2c shown in FIG. 3 has a structure that resistor R₅ is used in place of the current mirror circuit which is made up of the transistors Q₁₁, Q₁₂ and the constant current source I₄ in the squib driver circuit 2a described in FIG. 1. The resistor R₅ is connected between collector of the transistor Q₂ and collector of the transistor Q₁₄. According to this constitution, the circuit structure of squib driver circuit 2c is to be more simple than the squib driver circuit 2a shown in FIG. 1. Incidentally, in FIG. 3, the portions which correspond to ones in FIG. 1 are assigned the same reference numerals as ones of FIG. 1, and the descriptions of them will be omitted.

Preferred Embodiment 4

A squib driver circuit 2d shown in FIG. 4 has a structure that the leakage cut off resistor R_(LC) is connected in the same way as the preferred embodiment 2 between each base of transistors Q₁₃, Q₁₄ constituting the current mirror circuit and the power supply V_(CC) in the squib driver circuit 2c described in FIG. 3.

According to this constitution, the circuit structure of squib driver circuit 2d is to be more simple and a current consumption is to be small. Incidentally, in FIG. 4 the portions which correspond to ones in FIG. 3 are assigned the same reference numerals as ones of FIG. 3, and the description of those portions will be omitted.

Preferred Embodiment 5

A squib driver circuit 2e shown in FIG. 5 has transistors Q₁₇, Q₁₈ and two constant current sources I₅, I₅ connected as follows in place of the current mirror circuit consisting of transistors Q₁₁ -Q₁₄ and the current source I₄ in the squib driver circuit 2a described in FIG. 1. The transistor Q₁₇ has its emitter connected to base of transistor Q₇, its base connected to the node J_(A), and its collector connected to the ground. A constant current source I₅ is connected between emitter of the transistor Q₁₇ and the power supply V_(CC). The transistor Q₁₈ has its emitter connected to base of the transistor Q₈, its base connected to the node J_(B), and its collector connected to the ground. Another constant current source I₅ is connected between emitter of the transistor Q₁₈ and the power supply V_(CC). The emitter of the transistor Q₁₈ is connected to collector of the transistor Q₂. The transistor Q₂ has its emitter connected to the ground, and its base connected to the control signal input terminal T₆.

Because other remaining constitutions of the connections for transistor Q₁, transistors Q₃ -Q₁₀, capacitor C₁ and resistors R₁, R₂ etc. have the same constitutions as that of the squib driver circuit 2a in FIG. 1, the portions which correspond to ones in FIG. 1 are assigned the same reference numerals as ones of FIG. 1, and the description of these portions will be omitted.

The squib driver circuit 2e not only has an advantage according to the prior art squib driver circuit 2 described in FIG. 7 that the base current in the transistor Q₆ does not change before and/or after the removal of the current restriction, but also has a circuit structure which holds down the current consumption.

The operation in the squib driving circuit is as follows. Even if the removal signal input terminal T₆ for load current restriction operation is turned to logical "H" and the transistor Q₂ is switched on, current does not flow through the transistor Q₈, and only current from the constant current source I₅ is supplied to the transistor Q₂. Accordingly, even if said transistor is switched on, the increase of the current consumption is held down, base current in the transistor Q₆ does not change before and/or after the removal of the current restriction operation.

However, said squib driver circuit 2e is limited as follows in setting the potential on node J_(B) :

    potential on J.sub.B <V.sub.CC -V.sub.BE (Q.sub.18)-V(I.sub.5) min.,

provided that V(I₅) min. is a minimum voltage required to constitute the constant current source I₅.

For example, if the constant current source I₅ is constituted with a pnp transistor, at least 0.4 V is to be needed for the V(I₅) min., if V_(BE) (Q₁₈)=0.6, then the potential on node J_(B) =(V_(CC) -1) volt(s). Because of this, the resistance of shunt resistance R_(S) is determined.

Although, the present invention has been described in terms of preferred embodiments, it will be apparent to those of skill in the art that numerous variations and modifications may be made without departing from the true spirit and scope thereof, as set forth in the following claims. 

What is claimed is:
 1. A semiconductor integrated circuit comprisinga backup power supply means, an operational amplifier means for supplying a load with a load current from a power amplifier means and having a load current limit function for limiting the load current to provide an input stage with a negative feedback signal of a load current signal corresponding to the load current, and a load current limit function interruption means for interrupting the negative feedback operation of the operational amplifier means with an interrupting signal of the load current limit function, wherein the operational amplifier means and the load current limit function interruption means are backed up by the backup power supply means, the load current limit function interruption means consists of a constant current circuit means for controlling interruption of the negative feedback operation to supply the operational amplifier with a constant current.
 2. The semiconductor integrated circuit according to claim 1, wherein said constant current circuit means comprisesa constant current source, a first current control circuit for receiving a current from the constant current source and outputting a current corresponding to the current provided from the constant current source in accordance with the interruption signal, a second current control circuit for receiving an output current from the first current control circuit and making the negative feedback operation stop by supplying the operational amplifier with a current corresponding to the output current.
 3. The semiconductor integrated circuit according to claim 2, wherein said output current from the second current control circuit is supplied to an active load in a differential amplifier which constitutes an input stage of the operational amplifier.
 4. The semiconductor integrated circuit according to claim 3, wherein said first current control circuit and said second current control circuit consist of current mirror circuits respectively.
 5. The semiconductor integrated circuit according to claim 4, wherein said current mirror circuit consisting of the second current control circuit has a leakage cut resistor.
 6. The semiconductor integrated circuit according to claim 2, wherein said first current control circuit and said second current control circuit consist of current mirror circuits respectively.
 7. The semiconductor integrated circuit according to claim 6, wherein said current mirror circuit consisting of the second current control circuit has a leakage cut resistor.
 8. The semiconductor integrated circuit according to claim 2, wherein said first current control circuit consists of a resistor and said second current control circuit consists of a current mirror circuit.
 9. The semiconductor integrated circuit according to claim 8, wherein said current mirror circuit consisting of the second current control circuit has a leakage cut resistor.
 10. The semiconductor integrated circuit according to claim 3, wherein said first current control circuit consists of a resistor and said second current control circuit consists of a current mirror circuit.
 11. The semiconductor integrated circuit according to claim 10, wherein said current mirror circuit consisting of the second current control circuit has a leakage cut resistor.
 12. The semiconductor integrated circuit according to claim 2, wherein said current mirror circuit consisting of the second current control circuit has a leakage cut resistor.
 13. The semiconductor integrated circuit according to claim 1, wherein said constant current circuit means comprises two constant current sources, an amplifier having these constant current sources respectively as loads connected to inputs of differential amplifier constituting an input stage in the operational amplifier, a current from one of the constant current sources is bypassed by said interruption signal from an amplifier which is said one of the constant current sources as a load to cease the feedback operation by making the input stage of the operational amplifier out of balance.
 14. The semiconductor integrated circuit according to claim 1, wherein said output stage in the operational amplifier consists of a power MOSFET. 